The growing HIV pandemic and its increasing burden on women highlights the urgent need for novel safe and effective preventative strategies such as topical microbicides. We hypothesize that a truly safe and effective microbicide will require sustained, local delivery of a combination of drugs that target different steps in the HIV life cycle and a delivery system that addresses the challenges related to adherence. A safe microbicide should not interfere with host defenses nor increase the risk of selecting for drug resistant viruses. Achieving this goal requires more stringent pre-clinical and clinical evaluation of microbicides, focusing on their interactions with the genital tract mucosal environment. The goal of this iterative Program is to develop a combination microbicide delivered via intravaginal ring (IVR) to prevent the sexual transmission of HIV. This will be accomplished either through IVR formulation of a dual-active single agent, a pyrimidinedione, which is both a potent reverse transcriptase and entry inhibitor, or by combining two distinct antiretroviral (ARV) drugs. We will address the importance of the biological synergy between HIV and HSV by also combining acyclovir with ARV drugs to provide local sustained suppression of HSV. The safety of the microbicides and formulations will be explored using novel assays designed to assess their impact on the epithelial barrier and mucosal immunity. Exploratory clinical studies to evaluate the genital tract mucosal environment of Rwandan women will be conducted as defining the healthy genital tract environment will provide crucial information for establishing clinical biomarkers of microbicide safety. Pilot studies to examine the safety and tolerability of three different polyurethane rings will be conducted to identify the optimal composition for formulation of microbicide rings. Results of this study will guide the IVR formulation of our lead drugs, which will be advanced for further safety and regulatory studies including macaque PK and safety studies. A pre-Phase I clinical study is proposed to evaluate PK and safety of our lead IVR formulated microbicide and to determine if the released drug retains its antiviral activity using a spiking strategy.